An injection-molding machine including such an intermediate platen has been disclosed, for example, in commonly owned U.S. Pat. No. 4,207,051. This patent shows the intermediate platen slidably traversed by the tie bars interconnecting the two outer platens. Commonly owned U.S. Pat. No. 3,767,352 illustrates a stack mold whose inner plates are floatingly supported by the associated outer plates with the aid of an articulated linkage and without the use of a third platen. The latter arrangement is suitable only with mold plates of relatively low weight. Moreover, as likewise shown in the last-mentioned patent, the required precise alignment of the several plates in the mold-closed position necessitates the use of guide or leader pins on, say, the inner plates receivable in aligned bores of the adjoining outer plates. These guide pins, which must be of fairly large diameter, diminish the space available for mold cavities or cores.
Reference may further be made to commonly owned U.S. Pat. Nos. 3,659,997, 3,723,040 and 3,973,892 showing such inner plates directly supported by the tie bars. This type of mounting is applicable only to mold plates with guide bores conforming in size and location to the tie bars of a given machine. Moreover, the emplacement and removal of the mold plates requires a partial disassembly of the plate-supporting structure, such as a withdrawal of the tie bars from one of the platens, unless the mold plates themselves are split into separable sections.
A possible alternative already proposed is to suspend the inner mold plates on the upper tie bars by means of lateral extensions slidably engaging same. Such a system is relatively simple and quite satisfactory in terms of mold alignment but has only limited stability on account of the large offset of the points of support from the center of the plates; thus, unsymmetrical stresses acting upon these plates during the mold-opening and mold-closing strokes may result in dislocation and possible serious damage to the mold.
It is therefore advantageous to provide each inner pair of mold plates with its own platen in the general manner disclosed in the aforementioned U.S. Pat. No. 4,207,051. Since, however, such an intermediate platen cannot be readily removed from the machine when traversed by the tie bars as shown in that patent, the installation of the associated mold plates on its two surfaces is somewhat inconvenient and must be performed by a skilled operator.